This invention relates generally to semiconductor processing technology and more particularly the invention relates to dielectrics used in submicron devices and ULSI microelectronic circuits.
The metal-insulator-silicon (MIS) transistor including the metal-oxide-silicon (MOS) transistor is used in large scale integrated (LSI), very large scale integrated (VLSI), and ultra large scale integrated (ULSI) microelectronic circuits. The transistor has a current carrier source region formed in a surface of a semiconductor (e.g., silicon) body, a carrier drain region formed in the surface and spaced from the source, and between the source and drain is a channel region through which the current carriers flow. Overlying the channel region and aligned with edges of the source and drain is a gate electrode which is physically and electrically separated from the channel by a dielectric layer. Typically, the dielectric layer comprises a silicon dioxide (SiO.sub.2) and the gate comprises a doped polysilicon material.
To prevent migration of dopants such as boron into the silicon dioxide gate dielectric layer from the silicon substrate and from the doped polysilicon gate, nitride ions have been placed in the silicon oxide layer by ion implantation and by NH.sub.3 (anhydrous ammonia) nitridation. U.S. Pat. No. 5,397,720 discloses a method of making a MOS transistor having an improved oxynitride dielectric in which high quality ultra thin gate oxides have nitrogen ions therein with a profile having a peak at the silicon oxide-silicon interface. U.S. Pat. No. 5,578,848 discloses a low pressure rapid-thermal reoxidation of silicon nitride films with a rapid thermal reoxidation being carried out in N.sub.2 O or in O.sub.2 ambient.
Typical high thermal-budget oxynitridation processes, such as with N.sub.2 O or NO result in nitrogen incorporation at the silicon dioxide/silicon interface in relatively small amounts. Increasing nitrogen concentration improves reliability and the ability of the dielectric layer to suppress boron penetration, but increases fixed-charge and interface-trap density. This, in turn, degrades device performance by reducing the peak channel mobility and degrading transconductance of the MOS device. Furthermore, boron penetration into the silicon oxide with a diffusion barrier situated at the silicon dioxide-silicon interface can degrade the oxide reliability due to boron accumulation in the oxide. Ideally, therefore, it is desirable to have the nitrogen-rich layer located at the polysilicon/dielectric interface for an effective barrier to suppress boron diffusion from the gate without affecting the channel carrier mobility.
Heretofore, plasma nitridation has been used in the formation of 4 nm gate dielectric films with nitrogen at the top (gate electrode/dielectric) interface. The process consists of nitriding a previously formed thermal oxide with a remote, high density helicon-based nitrogen discharge at room-temperature for short durations on the order of a few seconds, followed by a high temperature post-nitridation anneal. Nitridation was performed at room-temperature by exposing the gate oxide to a short, high-density, remote helicon-based nitrogen discharge.